December 1, 2011

One of the greatest events in the history of Areiosan life was the advent of eukaryotic life. And this undoubtedly couldn’t have happened without the advent of mitochondria. Mitochondria are organelles within our cells that generate ATP, the energy currency of our cells. This process of ATP synthesis requires oxygen and produces carbon dioxide and water as waste products. Every complex animal on Earth uses this biochemical pathway by the virtue of their collective mitochondria’s electron transport chain. It is clear that without mitochondria, life on Earth would look very different. The origin of mitochondria on Earth as on Areios marks a profound threshold in the geological record; the rise of mitochondria presages the arrival of multicellular life.

Mitochondria in eukaryotic cells produce more energy through oxidative respiration than their anaerobic brethren, and multicellular life profits from this by investing that additional energy in development, growth, movement, and complexity. The number of mitochondria found in a eukaryotic is strong indicator of the kind of activity that specialized cell performs. Our muscle cells enjoy a much higher density of mitochondria than our blood cells, for instance. This is because our muscles cells require a tremendous amount of energy to move our bodies. Without mitochondria, complex life forms like animals almost certainly would have never emerged. Mitochondria in our cells are like microscopic power plants that provide energy for our cells. Without them, there simply isn’t enough energy in microbial metabolism to provide enough juice to power any complex animal.

Through this endosymbiotic event, all complex life was able to emerge. So the origin of mitochondria is fundamental to our understanding of the origin of complex life on Earth. As discussed in an earlier, researchers believe that a rickettsia bacterium was swallowed by another predaceous cell and survived; a mutualistic relationship occurred between the host and the rickettsia that evolved into the current configuration in our cells today. An archaean cell could have lost the genes that code for its cell wall. Without a cell wall to hold it back, this cell could live free with only a cell membrane keeping its cytoplasm separate from the rest of the outside world. This archaean cell would be able to fold onto itself and swallow other cells whole instead of filter-feeding organic molecules that floated nearby. Somehow, it ingested a live cell that didn’t get broken down by lysosomes or digestive enzymes. And this stowaway was useful; it could reduce any toxic oxygen molecules in the cytoplasm that would otherwise poison the archaean cell. This mutual relationship persisted for millennia until the engulfed cell became nothing more than a stripped-down set of genes responsible only for a select set of chemical pathways. This wasn’t the only endosymbiotic event in the history of Areios; there were three events that led to the rise of eukaryotic life.

There were two endosymbiotic events that transformed a new kingdom of life on Areios before the advent of mitochondria that led to animal life. First, a virus that failed to infect a bacterium evolved a mutualistic relationship that culminated in the creation of the first nucleus. The virus hijacked the cell’s DNA, but was unable to use the cellular machinery to replicate itself. Eventually, the virus’ protein coat served as a citadel for the cell’s genetic material against future viral sieges, making it harder for future viruses to infect this proto-eukaryotic cell. The virus’ protein coat would become the cell’s nucleus, directing the cell’s biological activities from a fixed, centralized location bound to the cell membrane. The second endosymbiotic event occurred when a sulfate-reducing organism was engulfed in another bacteria cell. Not only could organism detoxify the environment within the cell by converting sulfur compounds into useable forms, it could store those sulfur compounds internally and release them when food availability for its host was poor.

The final endosymbiotic event in Areiosan history created the mitochondria. This event parallels a similar that occurred on Earth; an organism that that can convert peroxides into water by reducing oxygen gets engulfed by a bacteria and sweeps the toxic peroxides out of the cell. Eventually, this process gets exploited by the host cell for making sugars because the process of reducing oxygen in the electron transport chain is so energetic. That electron transport chain in mitochondria is ubiquitous for animal life; it’s the power source for our ability to grow, move, reproduce, and maintain our cellular complexity.